Echelon is a global system of electronic eavesdropping used to intercept ordinary e-mail, fax, telex, and telephone communications carried over the world's telecommunications networks. Computerised systems target communications of interest by origin, destination, language or key words and are tagged and forwarded to the relevant intelligence agency (e.g. the US National Security Agency headquarters at Forte Meade, Maryland).

Unlike many of the electronic spy systems developed during the Cold War, Echelon is designed primarily for non-military targets such as governments, organisations, businesses and individuals in virtually every country in the world. It is not designed to monitor individual communications - it intercepts a whole mass of communications and sifts through them in 'real time' for the more interesting ones.

There is a chain of secret interception facilities around the world which tap into all the major components of the international telecommunications networks. Some monitor communications satellites, others land-based networks, and others radio commmunications and Echelon links all these facilities together.

Under ECHELON, a particular station's Dictionary computer contains not only its parent agency's chosen keywords, but also has lists entered in for other agencies. For example, the New Zealand's satellite interception station at Waihopai (in the South Island), the computer has separate search lists for the NSA, GCHQ, DSD (Defense Signals Directorate, Australia), and CSE (Communications Security Establishment, Canada) in addition to its own GCSB (Government Communications Security Bureau.

Whenever the Dictionary encounters a message containing one of the agencies' keywords, it automatically picks it and sends it directly to the headquarters of the agency concerned. No one in New Zealand screens, or even sees, the intelligence collected by the New Zealand station for the foreign agencies. Thus, the stations of the junior UK/USA allies (including Menwith) function for the NSA no differently than if they were overtly NSA-run bases located on their soil.

Menwith Hill assists in the interception of microwave communications by serving as a ground station for US electronic spy satellites. These intercept microwave trunk lines and short range communications such as military radios and walkie talkies.